1. Field of the Invention
This invention relates to a system for and a method of image processing where noises included in an image obtained by photographing are reduced, and a computer program for causing a computer to execute the method.
2. Description of the Related Art
There has been known a digital camera in which an infrared ray cut filter is retractably inserted into the optical path of a taking system such as a CCD to eliminate the light in an infrared region from the incident light so that the photographing can be carried out both with and without the infrared ray cut filter (See Japanese Unexamined Patent Publication No. 6 (1994)-141247). Since the CCD is sensitive to light in the infrared region, the reproduced color becomes reddish and cannot be faithful to the object when all the light from the object is imaged on the CCD. Accordingly, in such a camera, the photographing is carried out with the infrared ray cut filter in the normal photographing. Conversely, since the amount of the electric signal obtained from the CCD is reduced and S/N is reduced to largely increase the amount of noise if the amount of environmental light is poor and the amount of light impinging upon the CCD is reduced when the infrared ray cut filter is used. Accordingly, the photographing is carried out without the infrared ray cut filter in the photographing under a poor environmental light. Further, by handling an image obtained by such a photographing as a monochromatic image, reduction in the sensitivity is prevented by the use of light in a wavelength range invisible to the human eyes.
In the digital camera disclosed in Japanese Unexamined Patent Publication No. 6 (1994)-141247, use and unuse of the infrared ray cut filter are switched according to the amount of environmental light. However, in the digital camera disclosed in Japanese Unexamined Patent Publication No. 2004-260320, the use and the unuse of the infrared ray cut filter are switched according to information on the time, weather, halation and the like of the photographing. In the digital camera, the normal color photographing and the high-sensitive photographing can be automatically switched. Though the CCD has been increased in the number of pixels and the sensitivity, the problem of the noises included in an image obtained by photographing becomes more serious as the sensitivity becomes higher. Accordingly, there have been proposed various techniques to reduce the noises included in an image obtained by photographing. For example, there has been proposed a technique to reduce the noises included in an image obtained by detecting edges from the image obtained by photographing and carrying out the filtering with a low-pass filter to avoid the detected edges. With this technique, the noises included in an image obtained can be reduced without blurring the edges.
However, in the image obtained by the high-sensitive photographing, there are included very lots of noises and at the same time, the noise signal is sometimes larger in the amplitude than the edge signal included in the image, whereby it becomes difficult to distinguish the noise and the edge from each other. When the noise and the edge cannot be distinguished from each other, it is impossible to avoid the edges when the noises are reduced. Accordingly, the edges of the image can be blurred due to the noise reduction processing.